In many industries there is a need for large quantities of low cost and reliable sensing devices. As a specific example, the automotive industry requires a high volume of accelerometers used in ride control, inertial navigation, and crash sensing for airbag deployment. In each of these applications as well as many others, reliability, and low costs are key requirements. While large scale mechanical devices have long been known, they are not capable of meeting these requirements. Thus, micromechanical sensors such as pressure sensors and accelerometers are being actively pursued.
In sensors, as well as many other semiconductor devices, a major problem that arises is the mounting of the active die on a base to provide the required stability and the necessary means for further mounting the device in the final operating environment. Micromachined pressure sensors, for example, include a diaphragm or very thin major surface with a scribe grid, or window frame type, surrounding edge. The grid edges are then affixed to the surface of a base or mounting member with the diaphragm parallel and spaced from the mounting surface of the base. Generally, the base is designed to be affixed to a surface in the operating environment and is formed of material compatible with that environment, i.e. metal or the like. The problem is that the material of the base generally has a different temperature coefficient of expansion (TCE) than the semiconductor material of the diaphragm and surrounding grid. Thus, changes in temperature result in expansion and contraction of the grid with the base, which appear as changes in the pressure or other stress being sensed or, worse, result in physical damage to the sensor.
In an attempt to solve this problem, one prior art structure mounts the diaphragm and surrounding grid on the surface of the base using a flexible adhesive material, such as silicone gel or silicone RTV. The flexible adhesive is used to isolate the diaphragm and surrounding grid from the base so that stress in the base or differences in TCE are not transmitted to the sensor and sensed. These flexible adhesives raise another problem which is that they generally react badly to certain environments, e.g. silicon RTV swells in the presence of some solvents. Also, the flexible adhesive operates like a spring and eventually weakens wire bonds that are fixedly attached between the sensor and the base by ultrasonic action and the like. Thus, sensors using flexible adhesives are limited in the environments in which they can be used.
Other prior art methods of isolating the diaphragm and surrounding grid from stresses caused by TCE, is the use of tall glass pedestals of semiconductor material mounted between the grid and the base. However, such pedestals are expensive, because they use large quantities of semiconductor material and are hard to fabricate because of the longer port or tunnel connecting the diaphragm to the environment being sensed. Also glass tubes are used to convey the environment to the surface of the diaphragm, which tubes are again expensive because of the mounting and assembly problems.
Another problem arises, especially in pressure sensors, because pressure sensors must allow the environment inside of the package so that the pressure can be sensed. In corrosive environments, for example, this can be detrimental to sensing circuitry formed on the diaphragm. That is, the circuitry side of the diaphragm is susceptible to corrosion, contamination, and galvanic action. Attempts to form a barrier between the sensing circuitry and the environment always impact reliability and cost. One prior art structure, for example, employs a metal can type package with a corrugated stainless steel diaphragm and silicone oil to transmit pressure to the sensing die. In the above described flexible mounting scenario, the wires are covered by silicone gel in an attempt to keep ions off the die surface, which in turn creates high stresses on the wires due to thermal effects, air bubble formation in the gel, swelling caused by solvents, and die motion due to pressure cycles expanding and contracting the silicone RTV.
Further, as die area decreases, as it naturally will in the sensor field, the flexible adhesive becomes a narrow column which is even more unstable at the wire bonds, can result in die tilt at the bond between the die and the base, and is difficult to be kept open at the center of the die to allow for pressure sensing. This can be seen more clearly by noting that the flexible adhesive resembles a doughnut with a very small hole in the center.
It would be highly advantageous, therefore, to remedy the foregoing and other deficiencies inherent in the prior art.
Accordingly, it is an object of the present invention to provide improvements in mounting of semiconductor devices.
Another object of the invention is to reduce temperature sensitivity of surface micromachined sensors.
And another object of the invention is to provide improved methods of mounting semiconductor sensors to allow use of the sensors in virtually any environment.
A further object of the invention is to provide improved methods of mounting semiconductor sensors to allow for future reduction in size without altering the mounting structures.
A still further object of the invention is to provide improved methods of mounting semiconductor sensors to reduce stress, corrosion, and contamination of electrical circuits and leads of the sensors.